Abstract The interactions between the extremely shallow, funnel-shaped topography and dynamic processes in the North Branch (NB) of the Changjiang Estuary produce a particular type of saltwater intrusion, saltwater spillover (SSO), from the NB into the South Branch (SB). This dominant type of saltwater intrusion threatens the winter water supplies of reservoirs located in the estuary. Simulated SSO was weaker than actual SSO in previous studies, and this problem has not been solved until now. The improved ECOM-si model with the advection scheme HSIMT-TVD was applied in this study. Logarithmic and Chezy-Manning formulas of the bottom drag coefficient (BDC) were established in the model to investigate the associated effect on saltwater intrusion in the NB. Modeled data and data collected at eight measurement stations located in the NB from February 19 to March 1, 2017, were compared, and three skill assessment indicators, the correlation coefficient (CC), root-mean-square error (RMSE), and skill score (SS), of water velocity and salinity were used to quantitatively validate the model. The results indicated that the water velocities modeled using the Chezy-Manning formula of BDC were slightly more accurate than those based on the logarithmic BDC formula, but the salinities produced by the latter formula were more accurate than those of the former. The results showed that the BDC increases when water depth decreases during ebb tide, and the results based on the Chezy-Manning formula were smaller than those based on the logarithmic formula. Additionally, the landward net water flux in the upper reaches of the NB during spring tide increases based on the Chezy-Manning formula, and saltwater intrusion in the NB was enhanced, especially in the upper reaches of the NB. At a transect in the upper reaches of the NB, the net transect water flux (NTWF) is upstream in spring tide and downstream in neap tide, and the values produced by the Chezy-Manning formula are much larger than those based on the logarithmic formula. Notably, SSO during spring tide was 1.8 times larger based on the Chezy-Manning formula than that based on the logarithmic formula. The model underestimated SSO and salinity at the hydrological stations in the SB based on the logarithmic BDC formula but successfully simulated SSO and the temporal variations in salinity in the SB using the Chezy-Manning formula of BDC.

中文翻译：

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底部阻力系数对极浅河口咸水入侵的影响

摘要 长江口北支（NB）极浅的漏斗状地形与动力过程之间的相互作用产生了一种特殊类型的咸水入侵，即从北支到南支（SB）的咸水外溢（SSO）。 . 这种主要类型的盐水入侵威胁到位于河口的水库的冬季供水。在以往的研究中，模拟的 SSO 比实际的 SSO 弱，这个问题直到现在也没有得到解决。改进的 ECOM-si 模型与对流方案 HSIMT-TVD 应用于本研究。在模型中建立了底部阻力系数 (BDC) 的对数公式和 Chezy-Manning 公式，以研究对 NB 咸水入侵的相关影响。对2017年2月19日至3月1日在NB内的8个测量站采集的建模数据和数据进行比较，以及相关系数（CC）、均方根误差（RMSE）和技能3个技能评估指标水流速度和盐度的分数 (SS) 用于定量验证模型。结果表明，使用BDC 的Chezy-Manning 公式模拟的水流速度比基于对数BDC 公式的水流速度略准确，但后一个公式产生的盐度比前者更准确。结果表明，退潮时BDC随着水深的减小而增加，基于Chezy-Manning公式的结果小于基于对数公式的结果。此外，根据 Chezy-Manning 公式，大潮期间 NB 上游的陆上净水通量增加，并且 NB 的咸水入侵增强，特别是在 NB 上游。在NB上游的一个样带处，净样带水通量（NTWF）在大潮上游和小潮下游，Chez-Manning公式产生的值远大于对数公式的值. 值得注意的是，大潮期间基于 Chezy-Manning 公式的 SSO 是基于对数公式的 1.8 倍。该模型基于对数 BDC 公式低估了 SB 中水文站的 SSO 和盐度，但使用 BDC 的 Chezy-Manning 公式成功地模拟了 SSO 和 SB 中盐度的时间变化。